GtkDialog sets the default button to suggested when dialogs use header bars
If creating a GtkMessageDialog with a button
<action-widget response="yes" default="true">save_button</action-widget>
it will be displayed with the suggested
style, in contrast to GTK 3 where it does not have any particular style.
According to the HIG this should be only the case if the button in question is in a headerbar.
Edited by Alice Mikhaylenko